A method of preparing holographic optical elements or diffraction grating elements by utilizing the interference between leaser beams is one of well-known techniques commonly used for years. The diffraction gratings produced by the interfering laser exposure are applied to various devices, such as spectrometer, distributed feedback semiconductor lasers or fiber grating devices. The laser for use in the production process of such diffraction gratings includes a He—Cd laser, an argon ion laser, an excimer laser, a continuous wave laser or a nanosecond pulsed laser. These lasers can emit a beam with a relatively low energy density, and thereby a material to be processed is required to have photosensitivity. The inventors previously developed a method of producing holographic optical elements by utilizing the interference between femtosecond leaser beams.
In this method, the femtosecond leaser beams having a high energy density makes it possible to record a diffractive grating in almost all materials without the need for photosensitivity in a material to be processed. However, the femtosecond pulsed beam having an excessively high energy at a pulse peak causes increase in the non-linear optical interaction of the beam with air and a hologram-recording martial, resulting in difficulties in stably recording a hologram.
Particularly, in the process of forming an embedded hologram in the inside of a material, it is essential to propagate laser pulses with a high energy through the material, and the waveform of the laser pulses is liable to be irregularly deformed. Thus, in order to suppress such a distortion, it is required to select a specific material, or set the laser energy just below the threshold value for the encoding for recording a allowing the influence of the non-linear optical interaction to be ignored. However, the acceptable energy range of the laser pulse is narrow, and it is extremely difficult to record a hologram having a desired performance.